The present invention relates to a metal vapor discharge lamp having a translucent ceramic luminous tube and also to a method of producing such a metal vapor discharge lamp. More particularly, the invention is concerned with a luminous tube of the type mentioned above, provided with hermetic electrode supporting tubes attached to both ends thereof, as well as method of producing such a luminous tube.
In general, metal vapor discharge lamps having high luminous efficiency such as a high-pressure sodium lamp have a translucent ceramic luminous tube which is composed of a cylindrical ceramic tube member and ceramic or metallic end caps hermetically closing both ends of the ceramic tube member. The interior of the ceramic tube is charged with a metal such as mercury and sodium after evacuation.
Broadly, the method for hermetically sealing the luminous tube after charging with a metal can be sorted into two types: a method which does not make use of an exhaust tube and a method which makes use of exhaust tube.
In the first-mentioned method which employs no exhaust tube, a series of the steps such as evacuation of the interior of the ceramic tube member, charging with a metal and attaching of end caps to the ceramic tube member are conducted in a hermetic assembly room such as a bell-jar, employing a complicated assembly system. This method, therefore, is extremely difficult to conduct and can provide only a low efficiency of work.
For this reason, the second-mentioned method relying upon exhaust tube is more popular. FIG. 1 exemplarily shows a luminous tube with an exhaust tube. The luminous tube is composed of a ceramic tube member 1 and alumina end caps 2 and 3 attached to both ends of the ceramic tube member 1 by means of frit. Electrode supporting tubes 4 and 5 made of a heat-resistant metal such as niobium are fitted to the center of the end caps 2 and 3, respectively. The electrode supporting tubes 4 and 5 support respective electrodes at their inner ends which are projected into the tube member 1, thus serving as conductors for supplying electric power to the electrodes. One of the electrode supporting tubes, e.g., the electrode supporting tube 4, is intended for use as the exhaust tube, through which the interior of the luminous tube is evacuated and charged with a metal such as mercury and sodium. This electrode supporting tube 4, therefore, will be referred to as "exhaust electrode supporting tube", hereinunder.
This luminous tube, having an exhaust tube constituted by one of the electrode supporting tubes, is fabricated by the following method. As the first step, the electrode supporting tubes 4 and 5 are inserted into central holes formed in the alumina end caps 2 and 3. The electrode supporting tubes 4 and 5 are hermetically fixed to the alumina end caps 2 and 3 by means of a frit, simultaneously with the fixing of the alumina end caps 2 and 3 to the ceramic tube member 1. Subsequently, the outer end of the electrode supporting tube 5, which is not intended for use as the exhaust tube (referred to as "non-exhaust electrode supporting tube", hereinunder), is cut after a cold press-bonding followed by arc welding of the cut end as necessitated, thus forming an end seal 5' having a shape as shown in FIG. 1. Subsequently, the evacuation of the interior of the ceramic tube member 1 and the charging of the same with a metal are conducted through the exhaust tube constituted by the exhaust electrode supporting tube 4 and, thereafter, the outer end of the electrode supporting tube 4 is cold press-bonded and cut to form an end seal 4' in the same way as the sealing of the outer end of the electrode supporting tube 5.
When a luminous tube for a metal vapor discharge lamp such as a high-pressure sodium lamp is produced through the aid of the exhaust tube, the outer ends of the electrode supporting tubes are sealed by cold press-bonding followed by cutting, so that the end extremities 6 of these sealed ends have the form of blades as shown in FIG. 2A add are tipped to have reduced thickness as shown in FIG. 2B. In particular, both widthwise ends 6a, 6a of each sealed end have an extremely small thickness and, hence, are liable to be damaged, causing a risk of leak. Therefore, the evacuation and sealing operation, as well as the mounting of the luminous tube in an outer bulb, have to be done with greatest care.
When the end seals of the electrode supporting tubes is conducted by cold press-bonding, the sealing operation has to be done after mounting, on the alumina end caps, not only the exhaust electrode supporting tube but also the non-exhaust electrode supporting tube. In consequence, a laborious work is required for hermetically fixing the end caps to the ends of the ceramic tube member.
In order to obviate the above-described problems, the present inventors have already proposed an improved metal vapor discharge lamp in Japanese Utility Model Laid-Open No. 182359/1983. In this metal vapor discharge lamp, as shown in FIG. 3, the outer end of the non-exhaust electrode supporting tube 5 is sealed hermetically by fusing the tube material by other means than the cold press-bonding, e.g., by an arc discharge, thereby forming a hermetic sealed end 5". According to this method, the work for fusing and sealing the end of the non-exhaust electrode supporting tube 5 can be conducted without substantial difficulty and independently of the evacuation and charging of the interior of the ceramic tube member. In addition, since the attaching of the electrode supporting tube 5 to the associated end cap can be done after the sealing of the end of the tube 5, the tube 5 can be handled easily after it is hermetically attached to the end cap 3 of the ceramic tube member. In addition, the work for the assembly of the electrode supporting tube 5, the end cap 3 and the ceramic tube member 1 is facilitated, thus contributing greatly to the improvement in the efficiency of the work.
When the present inventors proposed this improved metal vapor discharge lamp, and even thereafter, it has been considered to be quite difficult to adopt the proposed sealing method to the sealing of the exhaust electrode supporting tube 4 and, therefore, the proposed sealing method has been applied only to the sealing of the end of the non-exhaust electrode supporting tube 5.
More specifically, the exhaust electrode supporting tube 4 has to be sealed in the final step of the production process after the charging with the metal in the ceramic tube member. In other words, this electrode supporting tube 4 cannot be sealed before the mounting on the associated end cap, unlike the non-exhaust electrode supporting tube 5.
When the exhaust electrode supporting tube is sealed by the fusing-type sealing method in the final step of the production process, the heat of the arc welding for fusing the end of the electrode supporting tube adversely affects the glass frit by which the end cap is hermetically fixed to the end of the ceramic tube member and also the charged metal carried by the electrode supporting tube. This problem is serious particularly in the case where the exhaust electrode supporting tube serves also as a metal reservoir in which the charged metal is reserved. Namely, in such a case, the charged metal is evaporated and scattered by the heat generated during the sealing operation, and is mixed into the fused end of the electrode supporting tube, causing troubles such as leak during operation of the luminous tube. In addition, since the charged metal absorbs impurity gases evaporated from the material of the electrode supporting tube, the purity of the charged metal is impaired to adversely affect the operation characteristics of the product luminous tube.
For these reasons, the sealing of the exhaust electrode supporting member has been conducted by cold press-bonding followed by cutting. The problem therefore still remains in connection with the likelihood of damaging of the sealed end 4' of the exhaust electrode supporting tube 4 and reliability is still low with regard to the sealed end 4', requiring a greatest care in the handling of the luminous tube after the sealing.
Another problem is encountered when the exhaust electrode supporting tube is used also as the metal reservoir. Namely, a temperature gradient appears during operation of the metal vapor discharge lamp such that the outermost end of the exhaust electrode supporting tube experiences the lowest temperature. The vapor pressure in the luminous tube and, hence, the lamp voltage are changed in relation to a change in this lowest temperature. This means that the length of projection of the exhaust electrode supporting tube beyond the end cap, which affects the temperature of the coldest outer end extremity of this electrode supporting tube, is a significant factor which determines the lamp voltage. Thus, the projection length has to be designed and selected in due consideration of the lamp voltage.
This, however, goes quite contrary to the demand from the view point of production. Namely, when the sealing of the exhaust electrode supporting tube is conducted by cold press-bonding after the mounting on the end cap, a considerable length of the electrode supporting tube has to be projected beyond the end cap, in order to prevent the juncture between the end cap and the electrode supporting tube from being affected by the deformation of the electrode tube end caused by the cold press-bonding. In addition, in order to make sure of the tight seal of the end of the electrode supporting tube by the cold press-bonding, the press-bonding has to be done over a substantial length. This means that the electrode supporting tube has to be prepared in a large length, resulting in an inefficient use of the expensive material such as niobium. Thus, the length of projection of the exhaust electrode supporting tube has to be determined also taking these factors into account.
Thus, when the electrode supporting tube is sealed by the cold press-bonding, it is quite difficult to determine projection length of the electrode supporting tube beyond the end cap on the basis of the lamp voltage solely, and the actual determination of the projection length encounters various restrictions.
It is to be pointed out also that, when the sealing is conducted by the cold press-bonding, the projection length and the shape of the electrode supporting tube fluctuate largely, resulting in fluctuation of the temperature at the coldest end of the electrode supporting tube, which in turn causes a variation in the lamp voltage of the metal vapor discharge lamp as the product.
The sealing of the end of the exhaust electrode supporting tube 4 by cold press-bonding causes also a problem in connection with a minute gap 7 which is formed in the sealed portion 4' as shown in FIG. 2B. Namely, during the operation of the lamp, the region around this minute gap 7 constitutes the coldest portion in the luminous tube, so that the charged metal such as sodium amalgam tends to invade this minute gap 7. The sodium amalgam thus trapped in the minute gap tends to evaporate as the lamp is started again but cannot evaporate perfectly. In consequence, the operation characteristics tend to be degraded particularly in the case of lamps in which the amount of the charged metal is small or in the case of so-called unsaturated-type sodium lamp.